The cell mebrane is largely impermeable to free ions. Honert (1937) proposed the carrier concept theory to explain uptake of ions across the cell membrane against the concentration gradient. He is of the opinion that cell membrane contains specific protein moleculcs which behave as carriers to pick up ions from outer “space. At first the carriers are activated.
The activated carrier then combines with the ions of outer space to form an intermediate external carrier-ion-complex. The carrier-ion-complex then moves across the cell membrane. On reaching the innerside of the membrane1the complex breaks down releasing the ions into inner space of the cell and the carrier gets separated.
The separated carrier again goes back to the outer surface to pick up fresh ions and the process of absorption of ions continues. Once the ions are released into inner space, they can not move out and then they are accumulated there. The metabolic energy in the form of ATP is supposed to be used in one or more steps of the process.
TIn the scheme, X & Y stands for carriers, X’ & Y’ for precursors of carriers, XK and YA for carrier ion complexes and k’ and A for cation and anion respectively. In the process carrier precursors x’ and y’ are activated to form carrier X and Y which combine with ions to form ion- carrier-complex XK and YA for carrying them to inner space and releasing them over there.
Factors affecting absorption of mineral salts:
Some of the important factors affecting absorption of mineral salts by plants are as follows:
(i) Temperature: The absorption of inorganic salts increases with an increase in temperature, but is confined to a very narrow range.
(ii) Oxygen: It greately influences active absorption of salts.
(iii) Light: Light enhance transpiration and effects photosynthesis, so exhibits it influence in salt absorption.
(iv) Hydrogen ion concentration: The decrease in the pH of soil solution accelerates the absorption of anions and increase in pH favours the absorption of cations.
(v) Presence of other ions: The rate of absorption of one type of ion is influenced by the presence of other ions in the medium. For example, rate of absorption of KT decreases if Ca++, MgT etc. are present in the medium. In otherwords the presence of Ca” antagonises the absorption of k*. This phenomenon is called ion-antagonism.
(vi) Growth: Active cell division, elogation and developmental processes promote absorption of salt.